Recently I have tried out a new monitor on my powermac. The monitor is a samsung 940BF and the video card is an ati radeon 9000 with ADC/DVI. With osx there is no issue with the monitor, so I assume both card and monitor are fine. With gentoo, I have a red flickering background. If I add a different resolution (any different resolution) and ctrl-alt-+ && ctrl-alt-- everything suddenly works.

Is there some setting in xorg.conf that is needed to properly use the DVI port on this video card? The ADC port works with no problems in either osx or gentoo.

I've got the same problem. After switching from VGA (with a DVI-to-VGA adapter) to DVI I have this red flickering. The work-around with the resoltion change works for me too, even with krandrtray.
The old monitor was a Sony GDM-F520 (CRT), the new one is a Fujitsu Siemens Computers (FSC) H22-1W (TFT).
Graphics card is a Radeon 9000 Pro.
Computer is a PowerMac G4 MDD (2003).

@Hopeless: There is no "MacModel" option described in neither the radeon nor the xorg.conf man page.

OT: In addition to this I have the problem, that the X server with the logon screen (kdm in my case) doesn't start again after log-out. What could be the problem here?

My kernel is now 2.6.24-r3 (latest stable ppc), 2.6.25-r6 (~ppc) and I also tried the state of the art 2.6.26, all gentoo-sources. The X.org X11 didn't change, it is 7.2 (x11-base/xorg-server-1.3.0.0-r6 and x11-base/xorg-x11-7.2), the X11 graphics driver “radeon” has recently been updated to 6.8.0 (x11-drivers/xf86-video-ati-6.8.0-r1).

According to this Debian article it could be a hardware limitation of DVI which has a limited bandwidth of 165 MHz, and high resolutions such as 1680x1050, which is the resolution I'm using for my TFT, push it to the limit.

This isn't very convincing to me because it works very well with Mac OS X 10.3.9 and with Mac OS X 10.5.1 to 10.5.4.

But still, the problem remains unsolved. At least for me.

My workaround is now to put a executable file, I named it fix-red-flickering.sh, in my ~/.kde/Autostart, which contains:

Code:

#!/bin/sh
xrandr -s 1024x768
xrandr -s 1680x1050

This sets the mode first to 1024x768 (you can use any mode that is defined in your /etc/X11/xorg.conf), and then returns it to 1680x1050. This way the red flickering is gone; but the main login screen (kdm in my case) has the flickering still.

At least Mac OS X isn't effected, so there has to be a way. And it works after resoltions changes, so it definitly has to work somehow...

Now, my question:
@davidgurvich: Do you still have this problem with DVI, like I do?

Only set-back: when I switch to a text console (e.g. Ctrl-Alt-F1) and then back (with Alt-F7) I have the red flickering again.

Anyway, if the ReducedBlanking doesn't work you may find it a good solution to put the xrandr-workaround in a startup-script of X.
I tried a few possibilities and found /usr/kde/3.5/bin/startkde to be a good choice.